The present invention relates to a laser scanning device that emits a scanning laser beam which is modulated in accordance with video signal or the like.
A laser scanning device has been widely employed in imaging apparatuses. Typically, the laser scanning device is configured such that a laser beam emitted by a laser diode is deflected using a polygonal mirror to scan a predetermined angular range. While the laser beam is deflected, emission of the beam of the laser diode is ON/OFF modulated in accordance with a video signal or the like. The scanning beam is incident on a surface such as a photosensitive surface to be scanned, which surface is moved in a direction perpendicular to the scanning direction, thereby a two-dimensional latent image being formed on the surface.
In such a laser scanning device, in order to compensate for an individual difference of light emission characteristics so that density thickness of the formed image is standardized, the output intensity of the laser diode is feed-back controlled. For this purpose, the laser scanning device is provided with an automatic power control (hereinafter referred to as an APC) circuit, which controls a driving current supplied to the laser diode in accordance with the intensity of the laser beam.
Recently, the laser scanning device is required to achieve a high-speed and high-quality imaging operation. In particular, as the imaging speed increases, a rising edge characteristic (i.e., a shape of the rising edge of the waveform representing the intensity of the laser beam) of the laser diode which is turned ON and OFF in accordance with an imaging pattern is required to be improved.
FIGS. 9A and 9B show a waveform of a driving current supplied to a laser diode, and a waveform showing the intensity of the laser beam emitted by the laser diode when the driving current shown in FIG. 9A is supplied. Generally, immediately after the driving current is supplied to the laser diode, the intensity of the laser diode does not respond to the driving current, and thus the rising edge of the waveform showing the intensity of the output laser beam inclines as shown in FIG. 9B. This delaying period tx (i.e., a delay from when the driving current is applied until the output intensity corresponds to the applied driving current) will be referred to as a rising delay hereinafter.
Due to the rising delay, an entire light amount (i.e., the quantity obtained by integrating the output intensity of the laser beam with respect to a time) is lower than an ideal case where the output intensity immediately corresponds to the applied driving current. When such a phenomenon occurs, since the entire light amount is lower than necessary, toner applied to the latent image is reduced, and thus, the image pattern exhibits insufficient density thickness. Further, the sharpness of the edge of the image pattern may be deteriorated.
In order to cope with the problem described above, there has been an improvement in which the level of the driving current is increased as indicated in FIG. 9C, in which a hatched area represents the increased current level. It should be noted that the light amount corresponding to the hatched area corresponds to the light amount lowered by the rising delay. With this configuration, even though the rising delay occurs, the entire light amount becomes sufficient.
FIG. 9D shows the same configuration as shown in FIG. 9C except that the light emitting period is longer. In this case, the increased amount (i.e., the hatched area in FIG. 9D) is much greater than the lowered light amount due to the rising delay, and the entire light amount during the light emitting period becomes larger than necessary. In this case, the density thickness of the image pattern may be too thick, and the consumption of the toner increases.
It may be possible to compensate for the lowered light amount due to the rising delay if the increased intensity level is varied depending on the light emitting period. However, such a control is very complicated to apply to the laser scanning device, and may not meet the requirement for the high-speed and high-quality imaging.